docs: def _get_stddev_rock(self, mag, imt):
          """
          Calculate and return total standard deviation for rock sites.
  
          Implements formulae from table 3.
          """
          C = self.COEFFS_ROCK_STDDERR[imt]
          if mag > C['maxmag']:
              return C['maxsigma']
          else:
              return C['sigma0'] + C['magfactor'] * mag